As a part of the goal of studying the mechanism of accumulation of cholesterol in atherosclerotic lesions, we are studying the incidence and accumulation of cholesteryl ester-rich cells, or foam cells, in spontaneous and experimentally induced aortic lesions of swine using flow cytometry and sorting. Flow cytometry is a technique in which measurement of fluorescence emission is carried out on fluorescently stained cells as they flow in suspension one at a time past a laser beam. Intimal-medial aortic tissue preparations have been enzymatically digested to provide the single-cell suspensions utilized in flow cytometry. A technique for fluorescent staining of the cholesterol ester-containing foam cells in these cell suspensions has been developed. After ethanol extraction of unesterified cholesterol, cells are treated with cholesterol esterase to convert esterified cholesterol to unesterified cholesterol. This unesterified cholesterol is then stained with filipin. Fluorescence microscopic examination of filipin-stained cell suspensions showed the presence of rare intensely stained cells among a majority of nonstaining cells. The intensely stained foam cells were of two types. One was round and contained many filipin-stained inclusions; the other was elongated and contained few filipin-stained inclusions. Preliminary flow cytometry and sorting experiments of these filipin-stained cell preparations showed that cholesterol ester-rich foam cells could be quantified and purified by sorting. Preliminary experiments indicate a positive correlation between macroscopic disease and the incidence of foam cells. Work will continue to complete the harvesting and analysis of cell suspensions prepared from aortic tissues of swine with spontaneous and experimentally induced atherosclerosis.